Individual atoms of oxygen are common in space, particularly around massive stars, but molecular oxygen has eluded astronomers until now.

The Herschel space observatory has located oxygen molecules in a dense patch of gas and dust. (Images: ESA/NASA/JPL-Caltech)
“Oxygen gas was discovered in the 1770s, but it’s taken us more than 230 years to finally say with certainty that this very simple molecule exists in space,” said Paul Goldsmith of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Goldsmith is the lead author of a recent paper describing the findings in the Astrophysical Journal. The Herschel mission is led by the European Space Agency, with contributions from NASA.

Astronomers searched for the elusive molecules in space for decades using balloons, as well as ground- and space-based telescopes. The Swedish Odin telescope spotted the molecule in 2007, but the sighting could not be confirmed. The molecules ultimately were discovered in the Orion star-forming complex.

The spectra recorded by Herschel reveal the signatures of oxygen molecules detected in the Orion nebula.
Goldsmith and his colleagues propose that oxygen is locked up in water ice that coats tiny dust grains. They believe that the oxygen detected by Herschel was formed after starlight warmed the icy grains, releasing water, which was converted into oxygen molecules.

“This explains where some of the oxygen might be hiding," Goldsmith said. "But we didn't find large amounts of it, and still don’t understand what is so special about the spots where we find it.”

The researchers plan to continue their hunt for oxygen molecules in other star-forming regions.

“Oxygen is the third most common element in the universe, and its molecular form must be abundant in space,” said Bill Danchi, Herschel program scientist at NASA headquarters in Washington. “Herschel is proving a powerful tool to probe this unsolved mystery. The observatory gives astronomers an innovative tool to look at a whole new set of wavelengths where the tell-tale signature of oxygen may be hiding.”

The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.